Glendora is a city in the San Gabriel Valley in Los Angeles County, California, 26 miles east of Los Angeles. As of the 2020 census, the …Glendora is a city in the San Gabriel Valley in Los Angeles County, California, 26 miles east of Los Angeles. As of the 2020 census, the population of Glendora was 52,558. World-famous Route 66 runs through the city.